To elucidate the sites and mechanisms of Ca and Mg absorption, and the effects of physiological and pharmacological factors on their transport, studies are proposed utilizing the technique of isolated rabbit tubule microperfusion. Unidirectional or bidirectional fluxes of calcium and magnesium will be studied using 45Ca, 47Ca, and 28Mg, and using 3H-Inulin or 125I-Glofil as volume markers. Studies will be performed on early and late proximal convoluted tubules, superficial and juxtamedullary pars recta, medullary and cortical thick ascending limb, and cortical and medullary collecting tubule. The effects of factors such as PTH, cyclic AMP, insulin, glucose, diuretics, mineralocorticoids and of alterations in the composition in the perfusion solution or of the bathing medium will be studied.